THALLOPHYTA: ALGAE 97 



tive spores. Its essential feature is the fusion of two cells to form a 

 zygote. It seems certain that originally gametes were derived from 

 vegetative zoospores that had become too small to form a new plant 

 directly. This is shown by the fact that in Ulothrix and many other 

 isogamous algae zoospores and gametes intergrade, the smaller spores 

 often germinating but producing dwarf filaments. The derivation of 

 gametes from zoospores is shown also by the striking similarity between 

 them in form and in the number and arrangement of their cilia. Any 

 peculiarity in the spores is duplicated in one or both of the gametes, as in 

 Oedogonium, the brown algae, etc. With few exceptions (notably the 

 Conjiigales, Charophyceae, and Fucales), gametic reproduction has not 

 replaced reproduction by vegetative spores but is supplementary to it. 

 In nearly all the green algae the zygote is a resting cell, accjuiring a heavy 

 wall and carrying the plant through a period of severe conditions into the 

 next growing season. In fact, the formation of gametes is often induced 

 by the advent of such conditions. In the brown and red algae the zygote 

 germinates at once. 



Originality, in the evolution of the algae, both of the fusing gametes 

 were ciliated and of the same size. This condition of isogamy is retained 

 by the yellow-green algae, diatoms, and many of the simpler green and 

 brown algae. In such forms as Pandorina, one of the pairing gametes is 

 slightly larger and less active than the other. In Cutleria both gametes 

 are ciliated, but the female gamete is considerably larger than the male. 

 In Dictyota and Fucus the female gamete (egg) is increased in size still 

 more and, although extruded into the water, is nonciliated, only the 

 male gamete (sperm) being motile. Finally, in Oedogonium and many 

 other algae, the large nonmotile egg is not liberated but is fertilized 

 within the oogonium by the small motile sperm. Thus the evolution of 

 heterogamy from isogamy has been gradual. 



After sexual reproduction had become established, one gamete retained 

 its motility and small size, while the other sacrificed its motility for an 

 increased nutritive capacity. The advantage of heterogamy lies in the 

 greater amount of reserve food that comes to be stored in the zygote. 

 This advantage is reflected by the occurrence of heterogamy in many 

 green algae, most brown algae, all stoneworts, and all red algae, as well 

 as in all plants above the thallophyte level. It should be emphasized 

 that heterogamy has arisen independently in the various groups of algae 

 where it occurs. 



The production of gametes in ordinary vegetative cells is characteristic 

 of Ulothrix, Oedogonium, and most other green algae. A more advanced 

 condition is seen in Vaucheria, the Charophyceae, and nearly all the 

 brown and red algae, where the gametes are borne in gametangia or sex 

 organs, which are specialized for reproduction, a function lost by the 

 other cells of the body. This tendency parallels the production of spores 



